Iron-Catalyzed Radical Allylic Substitution of Unprotected Allylic Alcohols

Allylic substitution reactions are essential in organic synthesis, enabling the transformation of allylic reagents into diverse alkenes. Traditional methods, which typically operate through ionic pathways, often require substrate preactivation to address high C─O bond dissociation energies, leading to challenges in regioselectivity and limited substrate compatibility. Here, we introduce an iron-catalyzed radical pathway for allylic substitution that directly activates unprotected allylic alcohols, leveraging the redox and oxophilic properties of low-valent iron to promote selective C─O bond cleavage and allylic transposition. This radical approach achieves high regio- and stereoselectivity, providing access to a broad array of di-, tri-, and tetra-substituted alkenes with moderate to excellent yields and exceptional E/Z selectivity. Mechanistic studies confirm that the iron catalyst generates radical intermediates and mediates efficient dehydroxylation, enabling this direct transformation without protective groups or Lewis acid activators. The method's versatility is demonstrated through a broad substrate scope, including complex natural derivatives and functionalized alkyl halides, along with successful gram-scale synthesis and downstream derivatization. This iron-catalyzed radical pathway offers a sustainable and efficient alternative to conventional ionic methods, expanding the scope of allylic substitutions and advancing radical-based methodologies in synthetic chemistry.

Mechanistic Insights into Copper-Catalyzed Asymmetric Cyanation of Allylic C-H Bonds

Direct C-H bond functionalization has emerged as one of the most powerful and practical strategies for the modification of drug molecules. We have recently disclosed a Cu/NFAS (NFAS = N-fluoroalkyl sufonamide) catalytic system that exhibits high site-, regio-, and enantioselectivity for the direct cyanation of allylic C-H bonds. Here, we present a mechanistic investigation of this catalyst system, including the elucidation of side reactions involved in the transformation. This work focuses on an in-depth analysis of the catalytic cycle based on kinetic studies by NMR spectroscopy and characterization of the catalyst speciation by EPR and UV-vis spectroscopy. These studies indicate that a fraction of NFAS is sacrificed to the side reactions of the Cu(II)-bounded N-centered radical (Cu(II)-NCR) species for the generation of silylated sulfonamides and (CN)2. The data also show a great dependence of the reaction yield and selectivity (hydrogen atom abstraction or HAA over side reactions) on the structure of the Cu(II)-NCR species. Kinetic studies and DFT calculations further reveal that oxidation of the CuCN species by NFAS, HAA process, and cyanation of Cu(II)-NCRs with TMSCN have comparable energy barriers, which collectively determine the rate of the overall C-H cyanation reaction.

Chemoselective Functionalization of Tertiary C-H Bonds of Allylic Ethers: Enantioconvergent Access to sec,tert-Vicinal Diols

While enantioenriched alcohols are highly significant in medicinal chemistry, total synthesis, and materials science, the stereoselective synthesis of tertiary alcohols with two adjacent stereocenters remains a formidable challenge. In this study, we present a dual catalysis approach utilizing photoredox and nickel catalysts to enable the unprecedented chemoselective functionalization of tertiary allylic C-H bonds in allyl ethers instead of cleaving the C-O bond. The resulting allyl-Ni intermediates can undergo coupling with various aldehydes, facilitating a novel enantioconvergent approach to access extensively functionalized homoallylic sec,tert-vicinal diols frameworks. This protocol exhibits nice tolerance towards functional groups, a broad scope of substrates, excellent diastereo- and enantioselectivity (up to 20 : 1 dr, 99 % ee). Mechanistic studies suggested that allyl-NiII acts as the nucleophilic species in the coupling reaction with carbonyls.

Direct Asymmetric α-Alkylation of β-Ketocarbonyl Compounds with Simple Olefins by Photoredox-Nickel-Hydrogen Atom Transfer Triple Catalysis

Although the asymmetric α-alkylation of carbonyl compounds with activated olefins has already been established, extending this methodology to less activated or nonactivated olefins remains a significant challenge due to the polarity mismatch in these ionic processes. An alternative approach involves the activation of the parent carbonyl compounds into electrophilic α-carbonyl radicals, which could potentially overcome this limitation. However, the lack of efficient catalytic systems has impeded the wide adoption of this strategy, particularly in realm of the catalytic asymmetric reactions. Here, we present a cooperative triple catalytic system that integrates photoredox, chiral Lewis acid, and hydrogen atom transfer (HAT) catalysts to achieve a direct asymmetric α-alkylation of β-ketocarbonyl compounds using simple olefins as alkylating agents. By combining a multifunctional chiral nickel Lewis acid with an iridium photoredox catalyst and a thiophenol catalyst under visible light, we have developed a highly efficient process that is temporally synchronized to facilitate a novel mechanism of electron and hydrogen transfer. This triple catalytic approach enables the intermolecular coupling of β-ketocarbonyl compounds with both less and non-activated olefins. This redox-neutral protocol provides an atom- and step-economic route to enantioselectively synthesize high-value molecules featuring an all-carbon quaternary stereocenter from feedstock chemicals, while only consuming photons.

Facile, general allylation of unactivated alkyl halides via electrochemically enabled radical-polar crossover

Electrochemically driven carbon-carbon formation is receiving considerable interest in organic synthesis. In this study, we present an electrochemically driven method for the formation of C(sp3)-C(sp3) bonds using readily available allylic carbonates, as well as primary, secondary, and tertiary alkyl bromides as electrophiles. This approach offers a highly selective route for synthesizing a broad range of allylic products with excellent functional group tolerance, all without the need for transition metal catalysts. Remarkably, this method also enables the smooth late-stage functionalization of various natural product- and drug-derived substrates, yielding the corresponding complex allylalkanes.

Highly Functional Allyl-Bicyclo[1.1.1]pentane Synthesis by Radical-Initiated Three-Component Stereoselective Allylation

Rapid access to highly functional allylated BCP synthons can be achieved with good selectivity and yield through a radical, three-component reaction (3CR) regime using various combinations of radical precursors and vinyl-appended heterocycles acting as versatile and modular precursors. This practical process combines mild operating conditions, a wide scope of reaction partners, and the ability to diversify the functionalized allylic scaffolds further using the allyl and other functional groups as synthetic branching points. The developed protocol allows structural alteration and increases the molecular complexity through late-stage drug modifications and drug conjugation approaches. Mechanistic probes demonstrate that the 3CR process is initiated by a selective, light-promoted radical addition to [1.1.1]-propellane, followed by coupling with the vinyl-substituted heterocycle, which represents a formal decarboxylative radical addition/double bond relay/protonation sequence.

Metallaphotoredox Enabled Single Carbon Atom Insertion into Alkenes for Allene Synthesis

Efficient methods for synthesizing allenes from readily available starting materials pose a persistent challenge in organic chemistry. In this work, we present a novel two-stage protocol for allene synthesis involving the single-atom insertion into alkenes, facilitated by synergistic photoredox and cobalt catalysis. Diverging from conventional methods such as the Doering-LaFlamme reaction, this photochemical rearrangement approach operates efficiently under mild conditions in a radical-based manner. The protocol exhibits a broad substrate scope and demonstrates applicability in the late-stage diversification of alkene-containing natural products and bioactive molecules. Preliminary mechanistic studies and density functional theory (DFT) calculations offer insights into the reaction pathway, indicating a radical mechanism involving fleeting cyclopropyl carbene intermediates followed by rapid ring opening to form allenes.

Enantioselective Three-Component α-Allylic Alkylation of α-Amino Esters by Synergistic Photoinduced Pd/Carbonyl Catalysis

Photoinduced excited-state Pd catalysis has emerged as an intriguing strategy for unlocking new reactivity potential of simple substrates. However, the related transformations are still limited and the enantiocontrol remains challenging. Organocatalysis displays unique capability in substrate activation and stereocontrol. Combination of organocatalysis and photoinduced excited-state Pd catalysis may provide opportunities to develop new enantioselective reactions from simple substrates. By applying cooperative triple catalysis including excited-state Pd catalysis, ground-state Pd catalysis, and carbonyl catalysis, we have successfully realized enantioselective α-allylic alkylation of α-amino esters with simple styrene and alkyl halide starting materials. The reaction allows rapid modular assembly of the three reaction partners into a variety of chiral quaternary α-amino esters in good yields with 90-99 % ee, without protecting group manipulations at the active NH2 group. The cooperation of the chiral pyridoxal catalyst and the chiral phosphine ligand accounts for the excellent chirality induction.

Dynamic Kinetic Activation of Aziridines Enables Radical-Polar Crossover (4 + 3) Cycloaddition with 1,3-Dienes

The cycloaddition of aziridines with unsaturated compounds is a valuable method for synthesizing nitrogen heterocycles. However, this process is predominantly substrate-controlled, posing significant challenges in regulating the regioselectivity of the C-N bond cleavage. In this study, we report a nickel-catalyzed dynamic kinetic activation strategy that enables catalyst-controlled activation of aziridines. Various types of aziridines, including 2-phenyl, 2-carbonyl, 2-alkyl, and disubstituted aziridines, consistently cleave their more sterically hindered C-N bonds to generate 1,3-radical anion intermediates. These intermediates participate in a highly regioselective 1,4-Heck/allylic substitution cascade with aromatic branched 1,3-dienes, resulting in a radical-polar crossover (4 + 3) cycloaddition that produces seven-membered azepine products. This approach not only complements traditional dipolar cycloaddition, in which aziridines typically act as zwitterionic 1,3-dipoles, but also introduces an unusual cycloaddition mode for 1,3-dienes. Experimental investigations and density functional theory (DFT) calculations provide insight into the reaction mechanism.

Photoinduced Pd-Catalyzed 1,4-Dicarbofunctionalization of 1,3-Butadienes via Aliphatic C-H Bond Elaboration

A three-component coupling strategy for 1,4-dicarbofunctionalization of 1,3-butadiene with C-H bearing substrates has been developed using photoinduced Pd catalysis, with aryl bromide serving as the hydrogen atom transfer (HAT) reagent. This photocatalytic coupling process achieves functionalized oxindole motifs in good yield and regioselectivity under mild reaction conditions. The versatility and synthetic utility of this method are demonstrated through the addition of a variety of C-H-bearing partners and various oxindole substrates to both substituted and unsubstituted butadiene.

Regioselective Alkylacylation of 1,3-Dienes by Merging N-Heterocyclic Carbene Catalysis with Photoinduced Palladium Catalysis

Herein, we develop a dual catalytic platform for the 1,2- or 1,4-alkylacylation reaction of 1,3-dienes with readily available alkyl halides and aldehydes by merging N-heterocyclic carbene catalysis with photoinduced palladium catalysis. A series of β,γ-unsaturated ketones are obtained in good to high yields. Mechanistic studies suggest that this reaction involves a radical process. The direct synthesis of flavanone from salicylaldehyde exemplified the potential capability of this dual catalytic platform.

Electrochemically Induced Synthesis of N-Allyloxyphthalimides via Cross-Dehydrogenative C-O Coupling of N-Hydroxyphthalimide with Alkenes Bearing the Allylic Hydrogen Atom

The electrochemically induced reaction between alkenes, bearing an allylic hydrogen atom, and N-hydroxyphthalimide was investigated. Cross-dehydrogenative C-O coupling with phthalimide-N-oxyl radical, derived from N-hydroxyphthalimide, occurs instead of oxidation of the allylic site, with the formation of a carbonyl group or functionalization of the double C=C bond. The discovered transformation proceeds in an undivided electrochemical cell equipped with a carbon felt anode and a platinum cathode. Coupling products were obtained with yields up to 79%. The developed process is based on the abstraction of hydrogen atom from the allylic position for functionalization while the C=C bond remains unreacted. The method exploits the ability of the phthalimide-N-oxyl radical to abstract hydrogen atoms with the following interception of the intermediate C-centered radical.

Recent Developments in the Metal-Catalyzed Synthesis of Nitrogenous Heterocyclic Compounds

Metal-catalyzed cyclization reactions have become a powerful and efficient approach for the stereoselective construction of both carbocyclic and heterocyclic ring systems. Transition metal complexes, with their ability to activate and selectively functionalize organic substrates, have revolutionized various areas of synthetic chemistry. This review highlights recent advancements in metal-catalyzed cyclization reactions, especially in the synthesis of nitrogen-containing heterocycles like imidazoles, pyridines, pyrimidines, and indoles. These advancements have significantly impacted fields such as natural product synthesis, pharmaceuticals, functional materials, and organic electronics. Novel catalytic systems, ligand designs, and reaction conditions continue to expand the capabilities of these reactions, driving further the progress made in synthetic organic chemistry. This review provides a comprehensive overview of recent research.

Copper-catalyzed yne-allylic substitutions: concept and recent developments

The catalytic (asymmetric) allylation and propargylation have been established as powerful strategies allowing access to enantioenriched α-chiral alkenes and alkynes. In this context, combining allylic and propargylic substitutions offers new opportunities to expand the scope of transition metal-catalyzed substitution reactions. Since its discovery in 2022, copper-catalyzed yne-allylic substitution has undergone rapid development and significant progress has been made using the key copper vinyl allenylidene intermediates. This review summarizes the developments and illustrates the influences of copper salt, ligand, and substitution pattern of the substrate on the regioselectivity and stereoselectivity.

Asymmetric Amination of Unstrained C(sp3)-C(sp3) Bonds

The asymmetric functionalization of unstrained C(sp3)-C(sp3) bonds could be a powerful strategy to stereoselectively reconstruct the backbone of an organic compound, but such reactions are rare. Although allylic substitutions have been used frequently to construct C-C bonds by the cleavage of more reactive C-X bonds (X is usually an O atom of an ester) by transition metals, the reverse process that involves the replacement of a C-C bond with a C-heteroatom bond is rare and generally considered thermodynamically unfavorable. We show that an unstrained, inert allylic C-C σ bond can be converted to a C-N bond stereoselectively via a designed solubility-control strategy, which makes the thermodynamically unfavorable process possible. The C-C bond amination occurs with a range of amine nucleophiles and cleaves multiple classes of alkyl C-C bonds in good yields with high enantioselectivity. A novel resolution strategy is also reported that transforms racemic allylic amines to the corresponding optically active allylic amine by the sequential conversion of a C-N bond to a C-C bond and back to a C-N bond. Mechanistic studies show that formation of the C-N bond is the rate-limiting step and is driven by the low solubility of the salt formed from the cleaved alkyl group in a nonpolar solvent.

Pd-Catalyzed 1,4-Difluoromethylative Functionalization of 1,3-Dienes Using Freon-22

We report a visible-light-driven, palladium-catalyzed 1,4-difluoromethylative functionalization of conjugated dienes using chlorodifluoromethane (ClCF2H, Freon-22) as a cost-effective difluoromethyl source. The excited palladium catalyst efficiently reduces the C-Cl bond, which generates a CF2H radical, followed by regioselective SN2' substitution to afford 1,4-difunctionalized products. This versatile, redox-neutral method accommodates diverse nucleophiles and exhibits broad functional group compatibility, making it suitable for late-stage functionalization in drug discovery and offering a direct route to difluoromethylated molecules.

Photoinduced Ligand-to-Copper Charge Transfer for Aryl Decarboxylative Allylation, Thiolation, and Bromination

Herein, aryl decarboxylative allylation, thiolation, and bromination reactions via photoinduced ligand-to-copper charge transfer are described. Utilizing inexpensive copper metal, the transformations of various aryl carboxylic acids enable the rapid synthesis of the corresponding alkene, thioether, and aryl bromide derivatives under visible light irradiation, which offers significant synthetic value. The reaction conditions are mild and straightforward, exhibiting a broad substrate compatibility. Furthermore, this method can be applied for the late-stage modification of complex drug molecules.

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Cobalt-Catalyzed Allylic Alkylation at sp3-Carbon Centers

The rising demand and financial costs of noble transition metal catalysts have emphasized the need for sustainable catalytic approaches. Over the past few years, base-metal catalysts have emerged as ideal candidates to replace their noble-metal counterparts because of their abundance and easiness of handling. Despite the significant advancements achieved with precious transition metals, earth-abundant cobalt catalysts have emerged as efficient alternatives for allylic substitution reactions. In this review, allylic alkylations at sp3-carbon centers mediated by cobalt will be discussed, with a special focus on the mechanistic features, scope, and limitations.

Transition Metal Mimetic π-Activation by Cationic Bismuth(III) Catalysts for Allylic C-H Functionalization of Olefins Using C═O and C═N Electrophiles

The discovery and utilization of main-group element catalysts that behave similarly to transition metal (TM) complexes have become increasingly active areas of investigation in recent years. Here, we report a series of Lewis acidic bismuth(III) complexes that allow for the catalytic allylic C(sp3)-H functionalization of olefins via an organometallic complexation-assisted deprotonation mechanism to generate products containing new C-C bonds. This heretofore unexplored mode of main-group reactivity was applied to the regioselective functionalization of 1,4-dienes and allylbenzene substrates. Experimental and computational mechanistic studies support the key steps of the proposed catalytic cycle, including the intermediacy of elusive Bi-olefin complexes and allylbismuth species.

Photoinduced Copper-Catalyzed Asymmetric Three-Component Radical 1,2-Azidooxygenation of 1,3-Dienes

Radical-involved multicomponent difunctionalization of 1,3-dienes has recently emerged as a promising strategy for rapid synthesis of valuable allylic compounds in one-pot operation. However, the expansion of radical scope and enantiocontrol remain two major challenges. Herein, we describe an unprecedented photoinduced copper-catalyzed highly enantioselective three-component radical 1,2-azidooxygenation of 1,3-dienes with readily available azidobenziodazolone reagent and carboxylic acids. This mild protocol exhibits a broad substrate scope, high functional group tolerance, and exceptional control over chemo-, regio- and enantioselectivity, providing practical access to diverse valuable azidated chiral allylic esters. Mechanistic studies imply that the chiral copper complex is implicated as a bifunctional catalyst in both the photoredox catalyzed azidyl radical generation and enantioselective radical C-O cross-coupling.

Metal-Free Decarboxylative Allylation of Oxime Esters under Light Irradiation

Allylation reactions, often used as a key step for constructing complex molecules and drug candidates, typically rely on transition-metal (TM) catalysts. Even though TM-free radical allylations have been developed using allyl-stannanes, -sulfides, -silanes or -sulfones, much less procedures have been reported using simple and commercially available allyl halides, that are used for the preparation of the before-mentioned allyl derivatives. Here, we present a straightforward photocatalytic protocol for the decarboxylative allylation of oxime esters using allyl bromide derivatives under metal-free and mild conditions. This methodology yields a diverse variety of functionalized molecules including several pharmaceutically relevant molecules.

Site-Selective Carbonylative Cyclization with Two Allylic C-H Bonds Enabled by Radical Differentiation

Controlling the site-selectivity of C-H functionalization is of significant importance and a formidable undertaking in synthetic organic chemistry, motivating the continuing development of efficient and sustainable technologies for activating C-H bonds. However, methods that control the site-selectivity for double C-H functionalization are rare. We herein report a conceptually new method to achieve highly site-selective C-H functionalization by implementing a radical single-out strategy. Leveraging the steric hindrance-sensitive CO-insertion as the radical differentiation process, a site-selective and stereoselective carbonylative formal [2 + 2] cycloaddition of imines and alkenes by sequential double allylic C-H bond activation was established without special and complicated HAT-reagents. This reaction was compatible with a wide range of alkenes and imines with diverse skeletons to deliver allylic β-lactams that are of synthetic and medicinal interest.

Selective 1,4-syn-Addition to Cyclic 1,3-Dienes via Hybrid Palladium Catalysis

1,4-cis-Disubstituted cyclic compounds play a pivotal role in pharmaceutical development, offering enhanced potency and bioavailability. However, their stereoselective and modular synthesis remains a long-standing challenge. Here, we report an innovative strategy for accessing these structures via mild conditions employing cyclic 1,3-dienes/alkyl(aryl)halides and amines. This procedure exhibits a wide substrate scope that tolerates various functional groups. The utility of this method is demonstrated in the efficient synthesis of a TRPV6 inhibitor, CFTR modulator, and other bioactive molecules. Combined experimental and computational studies suggest that the hybrid palladium-catalyzed radical-polar crossover mechanism is crucial for achieving exceptional 1,4-syn-addition selectivity (dr > 20:1).

An Expedient Radical Approach for the Decarboxylative Synthesis of Stereodefined All-Carbon Tetrasubstituted Olefins

We report a user-friendly approach for the decarboxylative formation of stereodefined and complex tri- and tetra-substituted olefins from vinyl cyclic carbonates and amines as radical precursors. The protocol relies on easy photo-initiated α-amino-radical formation followed by addition onto the double bond of the substrate resulting in a sequence involving carbonate ring-opening, double bond relay, CO2 extrusion and finally O-protonation. The developed protocol is efficient for both mismatched and matched polarity substrate combinations, and the scope of elaborate stereodefined olefins that can be forged including drug-functionalized derivatives is wide, diverse and further extendable to other types of heterocyclic and radical precursors. Mechanistic control reactions show that the decarboxylation step is a key driving force towards product formation, with the initial radical addition under steric control.

Catalytic Asymmetric Cascade Dearomatization of Indoles via a Photoinduced Pd-Catalyzed 1,2-Bisfunctionalization of Butadienes

Photoinduced Pd-catalyzed bisfunctionalization of butadienes with a readily available organic halide and a nucleophile represents an emerging and attractive method to assemble versatile alkenes bearing various functional groups at the allylic position. However, enantiocontrol and/or diastereocontrol in the C-C or C-X bond-formation step have not been solved due to the open-shell process. Herein, we present a cascade asymmetric dearomatization reaction of indoles via photoexcited Pd-catalyzed 1,2-biscarbonfunctionalization of 1,3-butadienes, wherein asymmetric control on both the nucleophile and electrophile part is achieved for the first time in photoinduced bisfunctionalization of butadienes. This method delivers structurally novel chiral spiroindolenines bearing two contiguous stereogenic centers with high diastereomeric ratios (up to >20 : 1 dr) and good to excellent enantiomeric ratios (up to 97 : 3 er). Experimental and computational studies of the mechanism have confirmed a radical pathway involving excited-state palladium catalysis. The alignment and non-covalent interactions between the substrate and the catalyst were found to be essential for stereocontrol.

Nickel-Catalyzed Four-Component Carbonylation of 1,3-Butadiene To Access β,γ-Unsaturated Ketones

A new strategy to obtain β,γ-unsaturated ketones via the cross-coupling of 1,3-butadiene, alkyl bromides, and arylboronic acids under 1 bar of CO with nickel as the catalyst has been developed. This newly developed four-component carbonylation procedure features advantages including using a cheap catalytic system, high step economy, mild reaction conditions, and excellent 1,4-regioselectivity, thereby providing a sustainable and alternative tool for β,γ-unsaturated ketones production compared to the present tactics. To elucidate the application potential of this method, olefin synthons are derived from the representative coupling product.

1,4-Aminoarylation of Butadienes via Photoinduced Palladium Catalysis

A visible-light-induced, three-component palladium-catalyzed 1,4-aminoarylation of butadienes with readily available aryl halides and aliphatic amines has been developed, affording allylamines with excellent E-selectivity. The reaction exhibits exceptional control over chemo-, regio-, and stereoselectivity, a broad substrate scope, and high functional group compatibility, as demonstrated by the late-stage functionalization of bioactive molecules. Mechanistic investigations are consistent with a photoinduced radical Pd(0)-Pd(I)-Pd(II)-Pd(0) Heck-Tsuji-Trost allylation cascade.

Interception of Transient Allyl Radicals with Low-Valent Allylpalladium Chemistry: Tandem Pd(0/II/I)-Pd(0/II/I/II) Cycles in Photoredox/Pd Dual-Catalytic Enantioselective C(sp3)-C(sp3) Homocoupling

We present comprehensive computational and experimental studies on the mechanism of an asymmetric photoredox/Pd dual-catalytic reductive C(sp3)-C(sp3) homocoupling of allylic electrophiles. In stark contrast to the canonical assumption that photoredox promotes bond formation via facile reductive elimination from high-valent metal-organic species, our computational analysis revealed an intriguing low-valent allylpalladium pathway that features tandem operation of Pd(0/II/I)-Pd(0/II/I/II) cycles. Specifically, we propose that (i) the photoredox/Pd system enables the in situ generation of allyl radicals from low-valent Pd(I)-allyl species, and (ii) effective interception of the fleeting allyl radical by the chiral Pd(I)-allyl species results in the formation of an enantioenriched product. Notably, the cooperation of the two pathways highlights the bifunctional role of Pd(I)-allyl species in the generation and interception of transient allyl radicals. Moreover, the mechanism implies divergent substrate-activation modes in this homocoupling reaction, suggesting a theoretical possibility for cross-coupling. Combined, the current study offers a novel mechanistic hypothesis for photoredox/Pd dual catalysis and highlights the use of low-valent allylpalladium as a means to efficiently intercept radicals for selective asymmetric bond constructions.

Rational design of super reductive EDA photocatalyst for challenging reactions: a theoretical and experimental study

We reported a novel electron-donor-acceptor (EDA) photocatalyst formed in situ from isoquinoline, a diboron reagent, and a weak base. To further optimize the efficiency of this photocatalyst, Density Functional Theory (DFT) calculations were conducted to investigate the substituent effects on the properties of vertical excitation energy and redox potential. Subsequently, we experimentally validated these effects using a broader range of substituents and varying substitution positions. Notably, the 4-NH2 EDA complex derived from 4-NH2-isoquinoline exhibits the highest photocatalytic efficiency, enabling feasible metal free borylation of aromatic C-H bond and detosylaion of Ts-anilines under green and super mild conditions. These experimental results demonstrate the effectiveness of our strategy for photocatalyst optimization.

Quaternary Carbon Synthesis by Titanocene Catalyzed Radical Allyl Transfer on Epoxides

A versatile titanocene-catalyzed radical allyl transfer reaction on epoxides is reported. Epoxide opening occurs regioselectively at the more hindered side, and variously substituted allyl sulfone may be coupled to this position in an efficient manner, enabling a rapid access to quaternary carbon centers with useful functionalities for further elaboration. Furthermore, the procedure can be expanded to stereoselective variants. This new radical allyl transfer expands the scope of allylation in organic synthesis.

Photocatalytic Enantioselective Hydrosulfonylation of α,β-Unsaturated Carbonyls with Sulfonyl Chlorides

This research explores the enantioselective hydrosulfonylation of various α,β-unsaturated carbonyl compounds via the use of visible light and redox-active chiral Ni-catalysis, facilitating the synthesis of enantioenriched α-chiral sulfones with remarkable enantioselectivity (exceeding 99 % ee). A significant challenge entails enhancing the reactivity between chiral metal-coordinated carbonyl compounds and moderate electrophilic sulfonyl radicals, aiming to minimize the background reactions. The success of our approach stems from two distinctive attributes: 1) the Cl-atom abstraction employed for sulfonyl radical generation from sulfonyl chlorides, and 2) the single-electron reduction to produce a key enolate radical Ni-complex. The latter process appears to enhance the feasibility of the sulfonyl radical's addition to the electron-rich enolate radical. An in-depth investigation into the reaction mechanism, supported by both experimental observations and theoretical analysis, offers insight into the intricate reaction process. Moreover, the versatility of our methodology is highlighted through its successful application in the late-stage functionalization of complex bioactive molecules, demonstrating its practicality as a strategy for producing α-chiral sulfones.

Visible-Light-Induced, Palladium-Mediated Desaturation/Sulfonation Cascade To Access 4-Sulfonyltetrahydropyridine Scaffolds

Herein, we report a visible-light-induced, palladium-catalyzed desaturation/sulfonation cascade, offering a concise route to a series of highly valuable 4-sulfonyltetrahydropyridine scaffolds from inexpensive and readily available piperidine derivatives with sodium sulfinates. The key to the success of this transformation is the well-designed sequence of palladium-mediated 1,5-hydrogen atom transfer/β-hydride elimination/allylic sulfonation process, which demonstrates the synthetic potentials for orchestrating synthetic events by rationally taking advantage of varied catalytic modes.

Robust Synthesis of Terpenoid Scaffolds under Mn(I)-Catalysis

The 6/6/5-fused tricyclic scaffold is a central feature of structurally complex terpenoid natural products. A step-economical cascade transformation that leads to a complex molecular skeleton is regarded as a sustainable methodology. Therefore, we report the first Mn(I)-catalyzed C(sp²)-H chemoselective in situ dienylation and diastereoselective intramolecular Diels-Alder reaction using iso-pentadienyl carbonate to access 6/6/5-fused tricyclic scaffolds. To the best of our knowledge, there is no such report thus far to utilize iso-pentadienyl carbonate as a substrate in C-H activation catalysis. Extensive mechanistic studies, such as the isolation of catalytically active organo-manganese(I) complexes, 1,3-dienyl-intermediates, and isotopic labeling experiments have supported the proposed mechanism of this cascade reaction.

New insights into the mechanism of synergetic photoredox/copper(i)-catalyzed carbocyanation of 1,3-dienes: a DFT study

This work presents a DFT-based computational study to understand the mechanism, and regio- and enantioselectivities in the synergetic photoredox/copper(i)-catalyzed carbocyanation of 1,3-dienes with alkyl redox-active esters. The calculated results show an unprecedented copper catalytic mechanism, where the reaction follows a catalytic cycle involving CuI-only catalysis, instead of a Cu(i)/Cu(ii)/Cu(iii)/Cu(i) cycle as proposed in the experimental study. Moreover, it is found that the critical step involves the reaction of the cyanocopper(i) species with an allyl cation rather than the cyanocopper(ii) species reacting with an allyl radical as proposed in the experiment, and that the photocatalyst is regenerated via single electron transfer from the allyl radical to the oxidized photocatalyst. In the newly proposed photoredox/copper(i) catalysis, the reaction consists of four stages: (i) generation of the copper(i) active catalyst, (ii) formation of an allyl radical with oxidative quenching of the photoexcited species, (iii) generation of an allylcopper complex accompanied by the regeneration of the photocatalyst, and (iv) formation of the allyl cyanide product with the regeneration of the copper(i) active catalyst. The cyanation of the allyl cation is calculated to be the regio- and enantioselectivity-determining step. The excellent regio- and stereoselectivities are attributed to the favorable CH-π interaction between the substrate and catalyst as well as the small distortion of the substrate.

A field guide to flow chemistry for synthetic organic chemists

Flow chemistry has unlocked a world of possibilities for the synthetic community, but the idea that it is a mysterious "black box" needs to go. In this review, we show that several of the benefits of microreactor technology can be exploited to push the boundaries in organic synthesis and to unleash unique reactivity and selectivity. By "lifting the veil" on some of the governing principles behind the observed trends, we hope that this review will serve as a useful field guide for those interested in diving into flow chemistry.

Metalloradical approach for concurrent control in intermolecular radical allylic C-H amination

Although they offer great potentials, the high reactivity and diverse pathways of radical chemistry pose difficult problems for applications in organic synthesis. In addition to the differentiation of multiple competing pathways, the control of various selectivities in radical reactions presents both formidable challenges and great opportunities. To regulate chemoselectivity and regioselectivity, as well as diastereoselectivity and enantioselectivity, calls for the formulation of conceptually new approaches and fundamentally different governing principles. Here we show that Co(II)-based metalloradical catalysis enables the radical chemoselective intermolecular amination of allylic C-H bonds through the employment of modularly designed D2-symmetric chiral amidoporphyrins with a tunable pocket-like environment as the supporting ligand. The reaction exhibits a remarkable convergence of regioselectivity, diastereoselectivity and enantioselectivity in a single catalytic operation. In addition to demonstrating the unique opportunities of metalloradical catalysis in controlling homolytic radical reactions, the Co(II)-catalysed convergent C-H amination offers a route to synthesize valuable chiral α-tertiary amines directly from an isomeric mixture of alkenes.

Iron-Catalyzed Coupling of Alkenes and Enones: Sakurai-Michael-type Conjugate Addition of Catalytic Allyliron Nucleophiles

The iron-catalyzed coupling of alkenes and enones through allylic C(sp3)-H functionalization is reported. This redox-neutral process employs a cyclopentadienyliron(II) dicarbonyl catalyst and simple alkene substrates to generate catalytic allyliron intermediates for 1,4-addition to chalcones and other conjugated enones. The use of 2,4,6-collidine as the base and a combination of triisopropylsilyl triflate and LiNTf2 as Lewis acids was found to facilitate this transformation under mild, functional group-tolerant conditions. Both electronically unactivated alkenes as well as allylbenzene derivatives could be employed as pronucleophilic coupling partners, as could a range of enones bearing electronically varied substituents.

Designed Iron Catalysts for Allylic C-H Functionalization of Propylene and Simple Olefins

Propylene gas is produced worldwide by steam cracking on million-metric-ton scale per year. It serves as a valuable starting material for π-bond functionalization but is rarely applied in transition metal-catalyzed allylic C-H functionalization for fine chemical synthesis. Herein, we report that a newly-developed cationic cyclopentadienyliron dicarbonyl complex allows for the conversion of propylene to its allylic C-C bond coupling products under catalytic conditions. This approach was also found applicable to the allylic functionalization of simple α-olefins with distinctive branched selectivity. Experimental and computational mechanistic studies supported the allylic deprotonation of the metal-coordinated alkene as the turnover-limiting step and led to insights into the multifaceted roles of the newly designed ligand in promoting allylic C-H functionalization with enhanced reactivity and stereoselectivity.

Interplay of diruthenium catalyst in controlling enantioselective propargylic substitution reactions with visible light-generated alkyl radicals

Transition metal-catalyzed enantioselective free radical substitution reactions have recently attracted attention as convenient and important building tools in synthetic chemistry, although construction of stereogenic carbon centers at the propargylic position of propargylic alcohols by reactions with free radicals remains unchallenged. Here we present a strategy to control enantioselective propargylic substitution reactions with alkyl radicals under photoredox conditions by applying dual photoredox and diruthenium catalytic system, where the photoredox catalyst generates alkyl radicals from 4-alkyl-1,4-dihydropyridines, and the diruthenium core with a chiral ligand traps propargylic alcohols and alkyl radicals to guide enantioselective alkylation at the propargylic position, leading to high yields of propargylic alkylated products containing a quaternary stereogenic carbon center at the propargylic position with a high enantioselectivity. The result described in this paper provides the successful example of transition metal-catalyzed enantioselective propargylic substitution reactions with free alkyl radicals.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Redox-Neutral 1,4-Dicarbonfunctionalization of 1,3-Butadiene by Merging Photoredox and Nickel Catalysis

The diverse functionalization of 1,3-butadiene provides wide applicability toward the synthesis of abundant and useful allylic compounds. Here, we describe a three-component and redox-neutral assembly of readily available C═X compounds, 1,3-butadiene, and various nucleophiles by merging photoredox and nickel catalysis, enabling the rapid synthesis of structurally diverse homoallyl amines and homoallylic alcohols.

Visible light-mediated NHC and photoredox co-catalyzed 1,2-sulfonylacylation of allenes via acyl and allyl radical cross-coupling

Visible light-mediated NHC and photoredox co-catalyzed radical 1,2-sulfonylacylation of allenes via cross-coupling between an allyl radical and an NHC-stabilized acyl radical.

Investigating Oxidative Addition Mechanisms of Allylic Electrophiles with Low-Valent Ni/Co Catalysts Using Electroanalytical and Data Science Techniques

The catalysis by a π-allyl-Co/Ni complex has drawn significant attention recently due to its distinct reactivity in reductive Co/Ni-catalyzed allylation reactions. Despite significant success in reaction development, the critical oxidative addition mechanism to form the π-allyl-Co/Ni complex remains unclear. Herein, we present a study to investigate this process with four catalysis-relevant complexes: Co(^MeBPy)Br₂, Co(^MePhen)Br₂, Ni(^MeBPy)Br₂, and Ni(^MePhen)Br₂. Enabled by an electroanalytical platform, Co(I)/Ni(I) species were found responsible for the oxidative addition of allyl acetate. Kinetic features of different substrates were characterized through linear free-energy relationship (Hammett-type) studies, statistical modeling, and a DFT computational study. In this process, a coordination-ionization-type transition state was proposed, sharing a similar feature with Pd(0)-mediated oxidative addition in Tsuji-Trost reactions. Computational and ligand structural analysis studies support this mechanism, which should provide key information for next-generation catalyst development.

Unexpected Copper-Catalyzed Cascade Reaction of 1,6-Enynes with Sulfoxonium Ylides

An unprecedented copper-catalyzed cascade reaction of 1,6-enynes with sulfoxonium ylides is reported, providing a series of structurally intriguing 2,3-disubstituted indolines bearing a conjugated dienone functionality at the 3-position in moderate to excellent yields with good chemo-, regio-, and diastereoselectivities under mild reaction conditions. Importantly, sulfoxonium-ylide-derived copper-carbene herein exhibits quite different reactivity from that of diazo copper-carbene. A rational mechanism, an initial ammonium ylide rather than allene formation, is proposed.

Exploiting photoredox catalysis for carbohydrate modification through C–H and C–C bond activation

Photoredox catalysis has recently emerged as a powerful synthetic platform for accessing complex chemical structures through non-traditional bond disconnection strategies that proceed through free-radical intermediates. Such synthetic strategies have been used for a range of organic transformations; however, in carbohydrate chemistry they have primarily been applied to the generation of oxocarbenium ion intermediates in the ubiquitous glycosylation reaction. In this Review, we present more intricate light-induced synthetic strategies to modify native carbohydrates through homolytic C-H and C-C bond cleavage. These strategies allow access to glycans and glycoconjugates with profoundly altered carbohydrate skeletons, which are challenging to obtain through conventional synthetic means. Carbohydrate derivatives with such structural motifs represent a broad class of natural products integral to numerous biochemical processes and can be found in active pharmaceutical substances. Here we present progress made in C-H and C-C bond activation of carbohydrates through photoredox catalysis, focusing on the operational mechanisms and the scope of the described methodologies.

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

Radical Termination via β-Scission Enables Photoenzymatic Allylic Alkylation Using "Ene"-Reductases

Allylations are practical transformations that forge C-C bonds while introducing an alkene for further chemical manipulations. Here, we report a photoenzymatic allylation of α-chloroamides with allyl silanes using flavin-dependent 'ene'-reductases (EREDs). An engineered ERED can catalyze annulative allylic alkylation to prepare 5, 6, and 7-membered lactams with high levels of enantioselectivity. Ultrafast transient absorption spectroscopy indicates that radical termination occurs via β-scission of the silyl group to afford a silyl radical, a distinct mechanism by comparison to traditional radical allylations involving allyl silanes. Moreover, this represents an alternative strategy for radical termination using EREDs. This mechanism was applied to intermolecular couplings involving allyl sulfones and silyl enol ethers. Overall, this method highlights the opportunity for EREDs to catalyze radical termination strategies beyond hydrogen atom transfer.